Joint Plant Ownership (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) operatingunit Turbines MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Jointly Owned Utility Plant Interests [Line Items]
Assets held for sale	$ 0	$ 3,080,000
Allowance for Funds Used During Construction
AFUDC equity recognized	2,300,000	3,300,000	$ 1,200,000
Other Regulatory Asset [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	$ 340,000	$ 2,013,000
Public Service Commission of Wisconsin [Member]
Allowance for Funds Used During Construction
Authorized AFUDC rate	50.00%	50.00%	50.00%
Authorized AFUDC rate - Signficiant projects (100%)	100.00%	100.00%	100.00%
MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Assets held for sale	$ 0	$ 3,080,000
Columbia Units [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	291,997,000	292,157,000
Accumulated depreciation	(105,778,000)	(94,766,000)
Property, plant, and equipment, net	186,219,000	197,391,000
Construction work in progress	1,777,000	2,021,000
Total property, plant, and equipment	$ 187,996,000	$ 199,412,000
Columbia Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, proportion of total net summer generating capability	29.00%
Jointly owned utility plant, plant capacity (in MW) | MW	217
Jointly owned utility plant, ownership interest	19.10%	19.40%
Assets held for sale	$ 0	$ 3,100,000
Jointly owned utility plant, fuel, operating, and maintenance expense	32,600,000	36,500,000	$ 36,200,000
Elm Road Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, fuel, operating, and maintenance expense	$ 19,700,000	17,600,000	17,300,000
Elm Road Units [Member] | MGE Power Elm Road [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, proportion of total net summer generating capability	14.00%
Jointly owned utility plant, proportion of total net summer generating capability, plant capacity (in MW) | MW	105
Jointly owned utility plant, number of operating units | operatingunit	2
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	$ 207,834,000	207,361,000
Accumulated depreciation	(37,071,000)	(33,897,000)
Property, plant, and equipment, net	170,763,000	173,464,000
Construction work in progress	468,000	530,000
Total property, plant, and equipment	$ 171,231,000	173,994,000
Elm Road Unit 1 [Member] | MGE Power Elm Road [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	615
Jointly owned utility plant, ownership interest	8.33%
Elm Road Unit 2 [Member] | MGE Power Elm Road [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	615
Jointly owned utility plant, ownership interest	8.33%
Forward Wind [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	$ 34,054,000	33,929,000
Accumulated depreciation	(13,413,000)	(12,530,000)
Property, plant, and equipment, net	20,641,000	21,399,000
Construction work in progress	45,000	67,000
Total property, plant, and equipment	$ 20,686,000	21,466,000
Forward Wind [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	18
Jointly owned utility plant, ownership interest	12.80%
Jointly owned utility plant, fuel, operating, and maintenance expense	$ 600,000
Forward Wind Turbines | Turbines	86
Forward Wind Proportionate Purchase Price	$ 23,000,000
Forward Wind Purchase Power Energy Output	12.80%
Forward Wind [Member] | MGE [Member] | Other Regulatory Asset [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets		600,000
Forward Wind [Member] | Joint Owners [Member]
Jointly Owned Utility Plant Interests [Line Items]
Forward Wind Aggregate Purchase Price	$ 174,000,000
West Campus [Member] | UW [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	45.00%
Jointly owned utility plant, fuel, operating, and maintenance expense	$ 6,600,000	6,300,000	$ 5,300,000
West Campus [Member] | MGE Power West Campus [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	55.00%
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	$ 113,259,000	113,328,000
Accumulated depreciation	(33,786,000)	(32,055,000)
Property, plant, and equipment, net	79,473,000	81,273,000
Construction work in progress	309,000	478,000
Total property, plant, and equipment	79,782,000	$ 81,751,000
Two Creeks [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Construction work in progress	43,900,000
Projected Solar Farm Project Costs	65,000,000
Allowance for Funds Used During Construction
AFUDC equity recognized	$ 1,400,000
Two Creeks [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	150
Jointly owned utility plant, ownership interest	33.00%
Two Creeks [Member] | MGE [Member] | Public Service Commission of Wisconsin [Member]
Allowance for Funds Used During Construction
Authorized AFUDC rate - Signficiant projects (100%)	100.00%
Badger Hollow I [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Construction work in progress	$ 18,700,000
Projected Solar Farm Project Costs	65,000,000
Allowance for Funds Used During Construction
AFUDC equity recognized	$ 300,000
Badger Hollow I [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW) | MW	150
Jointly owned utility plant, ownership interest	33.00%
Badger Hollow I [Member] | MGE [Member] | Public Service Commission of Wisconsin [Member]
Allowance for Funds Used During Construction
Authorized AFUDC rate - Signficiant projects (100%)	100.00%